The present invention pertains to methods and apparatus for optically scanning an object, such as a document, and particularly to an optical scanning apparatus, and an optical scanning method, which improve on prior art methods and apparatus for optically scanning an object. More specifically, the present invention is applicable to a document scanning apparatus, which is an apparatus configured to optically scan a document to thereby obtain a digital representation of the scanned document so that the digital representation of the document can be later used for various purposes. For example, the document scanning apparatus can be a flat-bed scanner which allows a user to capture a digital image of a document (or other object) placed on a scanning surface. The captured image can then be manipulated by a computer. For example, the scanned image can be cropped, rotated, or digital character recognition can be performed on text portions of the scanned image. Another example of an optical scanning apparatus which the present invention can be applied to is the document scanning section of a photocopier or a facsimile machine, or a combination printer-copier (or other so-called “all-in-one” document processing apparatus).
A document scanning apparatus generally includes a light bar assembly which includes a light source. Light from the light source reflects off of reflective portions of an object (such as a document) which is optically scanned by the light source. The light that is reflected from the object is recorded by sensors (which are typically located in the light bar assembly). Signals generated by the sensors in response to detecting the reflected light are then used by a processor to create an electronic image (typically in the form of a digital file) that is representative of the object being scanned. When the optical scanning apparatus comprises part of a document imaging apparatus which includes a sheet feeder for feeding one or more sheets of an original document to be scanned, then the light bar assembly can remain stationary as documents are moved past the light bar assembly by the sheet feeder. However, not all devices which use optical scanners have sheet feeders, and furthermore, there are instances where a sheet feeder is not helpful. For example, if a user wishes to scan one page of a book without removing the page from the book, then a sheet feeder is of no help (since the book cannot be fed through the sheet feeder). Accordingly, many optical scanning apparatus are provided with a drive system which is used to move the light bar assembly relative to the object to be scanned. Thus, an object can be placed on a scanning surface (for example, a glass surface, or a “platen”), and the light bar assembly can then be moved past the object using the drive system.
Turning to FIG. 1, a prior art optical scanning apparatus 10 is depicted in a plan view. The prior art scanning apparatus 10 includes a scanner body or chassis 12 which supports the other components of the scanner 10. Supported within the top of the scanner 10 is a platen (not shown to facilitate visualization of the other components), and mounted beneath the platen is a light bar assembly 14. A cover (not shown) is typically provided, which is used to cover the platen during a scanning process. The light bar assembly 14 is supported on two parallel guide rods 18, and is configured to move along rods 18 in directions “A” and “A′” to thereby move past a document (or other object) placed on the platen. Guide rods 18 must be accurately positioned relative to one another, and the platen, in order to allow the light bar assembly 14 to move beneath the platen in close proximity to the platen. At the same time, the light bar assembly 14 must form a close tolerance fit with the rods 18 so that excessive movement of the light bar assembly 14 (in directions other than the intended direction of travel) does not occur. Undesirable movement of the light bar assembly 14 while scanning a document can result in an inaccurate scanned image.
The light bar assembly 14 includes a light source 16, as well as other components (not shown), such as reflected light detecting sensors and electronic components for controlling activation of the light source and data routing of signals from the light detecting sensors. A flexible cable strip 38 is connected to the light bar assembly 14 and routes electrical power and activation signals to the light bar assembly. The flexible cable strip 38 is also used to route data signals from the light detecting sensors away from the light bar assembly 14 to a processor (not shown) so that the signals can be processed to generate an image of a scanned document or object.
The light bar assembly 14 is moved in directions “A” and “A′” by a drive system which includes the following components. Mounted on the chassis 12 is a spindle shaft 26. Spindle shaft 26 includes a first spindle barrel 28 and a second spindle barrel 30. The spindle shaft 26 includes a drive wheel 27. A motor 44 drives the spindle shaft 26 via a belt 46 which passes around the drive wheel 27. Mounted on a first side of the light bar assembly 14 is a two-wheel pulley 40, and mounted on a second side of the assembly 14 is a single wheel pulley 42. A pair of primary drive cables 20 are fastened to the first spindle barrel 28 at a first end of each primary drive cable (only one free end 23 of which can be seen in the view depicted). A first one of the primary drive cables 20 passes around the two-wheel pulley 40, and a second end 22 of this primary drive cable is then fastened to an anchor 24 which is attached to the chassis 12. The second primary drive cable 20 passes around a third pulley 44 (which is supported by the scanner body 12), and is then anchored at the second wheel of pulley 40. In this way, by driving the spindle shaft 26 in a first direction, the first of the two primary drive cables 20 is wound onto the first spindle barrel 28, thereby drawing the light bar assembly 14 in direction “A”. As the light bar assembly moves in direction “A”, the light source 16 can be used to optically scan a document) or other object) placed on a platen (not shown) set in the top of the chassis 12. By reversing the drive direction of motor 44, the second primary drive cable 20 can be wound onto the first spindle barrel 28, drawing the light bar assembly 14 back in direction “A′”. In this way, the light bar assembly 14 can be reset to the position depicted in FIG. 1 after a document (or other object) has been optically scanned. However, since stretching of the primary drive cables 20 (which can occur over time) can create a deadband space at the beginning of scanning, the drive cables 20 are typically manufactured from multiple strands of steel wire which are wrapped together to produce a single cable that is resistant to stretching.
The light bar assembly drive system can further include a second drive cable 32 which is anchored to the second spindle barrel 30 at a first end. The second drive cable 32 passes around the one-wheel pulley 42, and is anchored at a second end 34 of the cable to a second anchor 36. The second drive cable 32 can be used to facilitate movement of the light bar assembly 14 in direction “A”. In this way, more precise control of the movement of the light bar assembly 14 in direction “A” can be obtained. Precise control of the positioning of the light bar assembly 14 while optically scanning a document (or other object) is desirable since the data capture of the light reflections from the document or object being scanned needs to be precisely associated with position information. In this way an accurate image of the item being scanned can be generated.
The light bar assembly drive mechanism can, and typically does, also include a tensioning system to maintain tension on the drive belt 46 which connects the motor shaft 50 to the spindle shaft dive wheel 27. As depicted in FIG. 1, the tensioning system includes a spring 48 which is anchored to the chassis 12 at a first end, and is attached to a rigid arm 53 at a second end. The rigid arm 53 supports the drive shaft 50 of the motor 44. The motor 44 is mounted to a plate 51 which is in turn slidably supported by the chassis 12. Accordingly, as the length of the drive belt 46 changes (due to changes in temperature, or wear of the belt), the tensioning system maintains a constant force on the drive wheel 27. The tensioning system is an important component of prior art scanners since any slippage of the belt 46 on the drive wheel 27 will result in the light bar assembly 14 being moved in direction “A” at a speed which is different than the design speed. Since the rate of data collection is dependent on the velocity of the light bar assembly 14 as it moves in direction “A” to scan a document, a variance in speed of the light bar assembly 14 can affect the resultant scanned image.
As can be seen, the prior art light bar assembly drive system is quite complicated and includes a large number parts, many of which are moving parts, and many of which are precision parts and/or are prone to wear or deformation (typically stretching) over time. Consequently, prior art optical scanning apparatus are expensive to manufacture, prone to mechanical failure, difficult to repair, and susceptible of generating inaccurate scanned images when the drive system components get out of tolerance or out of alignment. Further, a large number of moving parts generally results in a fair amount of operating noise.
An additional problem with the prior art scanning apparatus depicted in FIG. 1 relates to the guide rods 18. As mentioned previously, the guide rods must form a precision fit with the light bar assembly 14, and therefore the rods 18 are typically precision machined from metal rods. While metal may be a desirable material of construction from the standpoint of providing a precision fit and resistance to wear, metal is particularly susceptible to thermal expansion. Thus, as the temperature within the scanning apparatus changes (which often is the result of use of the apparatus), the rods 18 will lengthen or shorten beyond an ideal design length. Ambient temperatures can also affect the length of the guide rods. This change in length of the guide rods can in turn affect the tension on the drive cables 20 and 32, as well as the length of the scanning path, all of which can affect the accuracy of the final scanned image.
FIG. 1 depicts only one embodiment of a prior art scanning apparatus. Other less complex light bar assembly drive systems are sometimes used. For example, the drive cable configuration (cables 20 and 32) can be replaced with one or two driven belts. However, as such belts are prone to elongation (stretching) over a period of time as a result of use and general aging, a belt tensioner (similar to the tensioning system depicted in FIG. 1) must be employed if any reasonable accuracy is to be maintained in the system. Further, unless the belt is manufactured from a particularly robust material, over time the belt can wear and eventually fail. As indicated, some of the detriments of using a belt to drive the light bar assembly can be reduced if a sufficiently robust belt is used. However, this can affect the overall size of the scanning apparatus. That is, a large scanner chassis may be required to provide additional space within the scanner to accommodate the oversized belt, while ensuring that the belt does not interfere with the optical scanning of a document. Typically, consumers do not want office equipment that is unduly large, and so increasing the size of the chassis to accommodate additional, or more robust, components is generally undesirable.
What is needed then is an optical scanning apparatus which achieves the benefits to be derived from similar prior art devices, but which avoids the shortcomings and detriments individually associated therewith.